A computer system requires the storage of large amounts of data. Such storage is usually provided by a magnetic disk drive that includes a magnetic disk, a slider where a magnetic head assembly including write and read heads is mounted, a suspension arm, and an actuator arm. When the magnetic disk rotates, air adjacent to the disk surface moves with the disk under the surface (ABS) of the slider, causing the slider to fly on an air bearing. The actuator arm swings the suspension arm to place the magnetic head assembly over selected circular tracks on the rotating magnetic disk, where signal fields are written and read by the write and read heads, respectively. The write and read heads are connected to processing circuitry that operates according to a computer program to implement write and read functions.
With reference to FIG. 1, an exemplary high performance read head employs a read element for sensing the signal fields from the rotating magnetic disk. The most prevalent sensor currently in use is the giant magnetoresistance GMR sensor also referred to as a spin valve. The GMR operates based on the spin dependent scattering of electrons through a conductive layer such as copper. A GMR sensor 100 includes a magnetically pinned layer 102, separated from a magnetically free layer 104 by a thin electrically conductive spacer layer 106. The pinned layer can include a pair of antiparallel pinned ferromagnetic layers 108, 110 separated by an antiparallel coupling layer 112, which can be constructed of for example Ru.
The magnetization of the pinned layer can be pinned as indicated by arrows 114, 116 by exchange coupling with an anitferrmagnetic material layer 118 such as PtMn. While antiferromagnetic materials are not magnetic in and of themselves, when exchange coupled with a ferromagnetic material, they can strongly set the magnetization of the ferromagnetic material.
The free layer 104 has a magnetization that is biased in a direction 120 parallel with an ABS surface of the slider in which it is incorporated. Although the magnetization is biased in the direction indicated by arrow 120, the magnetization of the free layer is free to rotate in response to a magnetic field. As those skilled in the art will recognize, the relative directions of magnetization of the free 104 and pinned 102 layers affects the electrical resistance through the sensor. This resistance is greatest when the magnetization of the free layer 104 and the ferromagnetic layer 114 of the pinned layer 102 are antiparallel and is at a minimum when they are parallel. These magnetizations 120 and 114 are set to be perpendicular in the absence of a magnetic field, so that the signal sensed from relative movements about this quiescent state will be at its most linear relationship.
The magnetization of the free layer in a typical magnetoresistive sensor is biased by hard bias layers 122, 124, disposed at either lateral edge of the sensor. Electrically conductive leads 123, 125 disposed over the bias layers provide electrical sense current to the sensor 100 during use. First and second electrically insulating gaps 100, 129, insulate the sensor 100 from a pair of magnetic shields 131, 133. A capping layer 135, such as Ta, can also be provided to prevent corrosion during subsequent manufacturing processes.
The bias layers 122, 124 are constructed of a hard magnetic material (ie. a material with a high coercivity), and are magnetized by placing them in a magnetic field. The amount of biasing achieved by the hard bias layers 122, 124 is critical in that if the free layer is too strongly biased, the sensor will not be sufficiently sensitive. In other words, it would require a very strong field to rotate the direction of magnetization 120 of the free layer. The sensor would therefore be useless. On the other hand, weak biasing of the free layer results in domain fluctuations in the free layer resulting excessive noise in the signal read from the sensor. This too renders the sensor useless.
With reference now to FIG. 2, the relationship between bias strength and sensor reliability can be understood more clearly. More specifically, the curve 202 illustrates the relationship between the head amplitude, ranging from 400 μV to 3000 μV, and the soft error rate. The strength of the bias field is inversely proportional to the amplitude level. Those skilled in the art will recognize that manufacturing processes and materials are not perfect and that actual bias strength as generated by a hard bias layer will vary from wafer to wafer and even from head to head within a given wafer. As the bias strength drops below an acceptable level such as for example, point 206, where the amplitude is high, the free layer will become unstable, such that heads in this range will be unusable. Conversely, as the bias strength increases, heads falling to the left of a given point 204 on the curve 202, where the amplitudes are low, will be unacceptable, because the free layer will be too insensitive and unable to detect fields.
Therefore, there is a strong felt need for reliable free layer biasing in a magnetoresistive sensor. Such biasing would preferably always, or nearly always maintain the bias field within an acceptable range, eliminating the need to scrap heads due to insensitivity or instability.